This proposal deals with the investigation of the mechanism of HCl secretion in frog gastric mucosa, and the mechanisms of Na ion and Cl ion transport in rabbit descending colon. These ionic transport processes will be studied using transepithelial and intracellular electrical impedance measuring methods, coupled with equivalent circuit analysis using morphologically-based models of the tissues. Previous studies have been unable to yield conclusive results due to three major problems: (1) The inability to separate transepithelial measurements into the effects of each of the different epithelial membranes (the apical and basolateral membranes, and the tight junctions). (2) The inability to quantitate membrane area and area changes. And (3) the inability to model effectively the morphological complexities in the tissues by equivalent circuits. The use of impedance techniques will solve these problems. In gastric mucosa, the following questions will be addressed regarding HCl secretion. Does the process involve an electrogenic (current carrying) or electroneutral (cotransport or ion exchange) process? Is HCl secretion coupled to an observed Cl ion secretion process? Does an observed apical area transformation reflect an exocytotic secretion mechanism? In descending colon the following questions will be addressed regarding Na ion and Cl ion transport. Are these processes mediated by selective membrane ionic conductance properties? Does the tight-junctional conductance pathway play a role in these processes? Are the two processes regionally segregated to different cell types in the tissue (e.g., surface and crypt regions)?